An Organic Light-Emitting Diode (OLED) display, also known as an organic Electroluminescent (EL) display, is a novel flat-panel display device. Having advantages of simple fabrication process, low cost, low power consumption, high luminous brightness, adaptable operation temperatures, light and compact, fast response, easily achieved in colorful displays and large screen displays, easily compatible with Integrated Circuit (IC) drivers, and easily implemented as flexible displays, OLED displays have a bright application prospect.
As illustrated in FIG. 1, a display device in the OLED display generally comprises a base substrate 1, a semiconductor layer 2 (also known as an ITO layer) and an electrode layer 6 sequentially disposed on the base substrate 1, as well as structural layers disposed between the semiconductor layer 2 and the electrode layer 6. Herein, the semiconductor layer 2 is connected to a positive terminal of a power and functions as an anode, the electrode layer 6 is connected to a negative terminal of the power and functions as a cathode. The structural layers comprise a Hole Transmission Layer (HTL) 3 connected to the semiconductor layer 2, an Electron Transmission Layer (ETL) 5 connected to the electrode layer 6, and an EL layer 4 disposed between the HTL 3 and the ETL 5. When an appropriate voltage is applied to the semiconductor layer 2 and the electrode layer 6, holes generated by the anode and charges generated by the cathode will combine with each other in the EL layer 4 to produce light; moreover, three primary colors of red (R), green (G), blue (B) are respectively produced based on different schemes, to form basic colors.
Due to that the electrode layer 6 is generally made of a metal, it has a high reflectivity; as a result, when the OLED display is used outdoors under a strong ambient light, the reflected light will make the outdoors readability relatively poor. Currently, a circular polarizer is normally used to solve the above problem. As illustrated in FIGS. 2 and 3, the circular polarizer 7 is disposed on a surface of the base substrate 1 that is far from the electrode layer 6, and the circular polarizer 7 sequentially comprises a protection layer 701, a polarizing layer 702 and a quarter-wave plate 703 along an incident direction of the ambient light; wherein the quarter-wave plate generally employs a uniaxial phase retarder, normally with a refractive index factor Nz of 0 or 1.
When the ambient light is incident on the polarizing layer 702 vertically after passing the protection layer 701, light in one of the polarization directions is absorbed, and linearly polarized light having a polarization direction the same as the transmission axis of the polarizing layer 702 is transmitted. The linearly polarized light is turned into a left- or right-handed circularly polarized light after passing the quarter-wave plate 703 having an angle of 45° with the circularly polarized light. After being reflected by the electrode layer 6, the left- or right-handed circularly polarized light is turned into a right- or left-handed circularly polarized light with an opposite rotation direction and then turned into a linearly polarized light with a polarization direction the same as the absorbing axis of the polarizing layer 702 upon passing the quarter-wave plate 703 for a second time. The linearly polarized light is therefore absorbed by the polarizing layer 702, thereby preventing the ambient light from being reflected and improving the outdoor readability.
The circular polarizer absorbs almost all of vertically incident ambient light. However; for obliquely incident ambient light, in its polarization plane, the direction of the optical axis of the quarter-wave plate 703 or the transmission axis of the polarizing layer 702 will be deflected to a certain degree, which will cause the angle between the optical axis of the quarter-wave plate 703 and the transmission axis of the polarizing layer 702 to be changed, thereby causing light leakage. As illustrated in FIG. 4, the circular polarizer hardly transmits vertically incident ambient light; in contrast, it have a maximum light leakage rate of up to 5% for obliquely incident ambient light. Assuming luminous intensity per unit area for the Sun is 20000 nit, when the maximum light leakage rate of the circular polarizer for obliquely incident ambient light is 5%, light intensity of the reflected light may be up to 1000 nit, which will severely harm the outdoor readability of the OLED display. Moreover, the stronger the ambient light is, the poorer the outdoor readability of the OLED display is (in FIG. 4, azimuth angle is distributed along the outer circumference, and polar angle is distributed along the radius; and the relative light intensity is zero at locations having highest gray scale level. The lower the gray scale level is, the higher the relative light intensity is).
In the following, the principle of light leakage for the circular polarizer will be explained with reference to the Poincaré Sphere in which two axes S1 and S2 normal to each other are provided.
As illustrated in FIG. 5, when the ambient light is incident on the circular polarizer vertically, that is, when viewed from the normal line direction of the circular polarizer, the transmission axis of the polarizing layer 702 is normal to point A. Point A is an intersection point between the positive direction of the axis S1 and the circumference, and the optical axis 703A of the quarter-wave plate 703 coincides with the axis S2. In this case, the light incident on the circular polarizer is totally absorbed when passing the circular polarizer for a second time after being reflected by the electrode layer; thereby no light leakage will be caused.
As illustrated in FIG. 6, when the ambient light is incident on the circular polarizer along a direction having an azimuth angle of 0° and a polar angle of about 60°, that is, when viewed from an oblique direction of the circular polarizer, the transmission axis of the polarizing layer 702 is not deflected, and is still normal to the intersection point between the positive direction of the axis S1 and the circumference, while the direction of the optical axis 703A of the quarter-wave plate is deflected. In this case, the light incident on the circular polarizer cannot be totally absorbed when passing the circular polarizer for a second time after being reflected by the electrode layer, thereby causing light leakage. Based on symmetry of optical structures, light leakage will be caused as well when the azimuth angle is respectively 90°, 180° and 270°.
As illustrated in FIG. 7, when the ambient light is incident on the circular polarizer along a direction having an azimuth angle of about 45° and a polar angle of about 60°, that is, when viewed from another oblique direction of the circular polarizer, the direction of the optical axis 703A of the quarter-wave plate is not deflected, and still coincides with the axis S2, while the transmission axis of the polarizing layer 702 is deflected, that is, although the transmission axis of the polarizing layer 702 is still perpendicular to point A, point A is deflected from the intersection point between the positive direction of the axis S1 and the circumference. Such a case will cause light leakage too. Based on symmetry of optical structures, light leakage will be caused as well when the azimuth angle is respectively 135°, 225° and 315°.